Fix some instabilities in a few networking tests

There's one remaining test that still uses the wildcard (SocketImplCombinations). It has been observed failing in the past, so it should be good to get it fixed.
The UnreferencedMulticastSockets which has also been observed failing could also be updated with some more diagnosis.
Finally, the DigestEchoServer$TunnelingProxy class has also been observed causing some instabilities in the httpclient tests that use it: the proxy tunnel doesn't properly propagate errors from downstream to upstream, and conversely, which sometimes leaves the reader waiting for data that will never come.